1. The Field of the Invention
This invention relates to systems and methods for transferring data between a sending system and a receiving system. More particularly, the present invention relates to high speed transmission of information over band-limited channels.
2. The Prior State of the Art
The time to send information from one place to another was, at one time, measured in days or weeks. Today, however, an ever increasing array of technologies have reduced the time to minutes or seconds. In the United States, nearly every home has a telephone that allows instant voice and data communication with virtually any location within the United States and most industrialized countries. Wireless and cellular communication devices allow contact with individuals in virtually any location. Networks allow almost instantaneous communication between computers, whether separated by a few feet or vast geographic distances. The Internet, a global network of computers, provides an almost endless variety of information that can be accessed from a telephone line virtually anywhere. Satellites and other wireless technology also provides a wide variety of information including entertainment, communication, and other services.
As the need and desire to transfer information through a wide variety of networks has increased, the burden on the networks has dramatically increased. For example, although the Internet started as an experimental project to communicate information between large research centers, many homes and businesses today have accessed to, and regularly use, the Internet to transfer vast amounts of information. This has, in turn, created burdens on the public switched telephone network (PSTN) that were unenvisioned when the network was constructed. In many areas, the increased burden has resulted in an insufficient capacity to carry both the data traffic and normal telephone traffic. Many companies and individuals have severely criticized telecommunications service providers, calling again and again for an end to the bandwidth bottlenecks in their networks. Given the need and interest in data communications, many techniques have been developed for compressing data bandwidth in communication systems in order to squeeze as much information through a bandwidth limited network as possible.
The PSTN, which carries a large percentage of the telecommunication traffic today, has been constructed with a combination of fiber optic cables and copper wire cables. The communication bandwidth of a fiber optic cable, generally employed as trunk lines, is much greater than the communication bandwidth of the copper wire that is generally employed to an individual home. While theorists conjecture that future techniques, such as wavelength division multiplexing, may allow a single fiber optic cable to someday transport up to twenty-five terabits of data per second, such a solution is a very long ways off. Costs, regulatory uncertainty, operational issues, and questions regarding real demand are all slowing the push for large bandwidth communication networks in the mass market. Some experts have projected that it will take at least twenty years to fully replace the copper wiring in today's telephone network with fiber optic cable. Thus, there currently exists a large need for squeezing as much data through a typical copper cable utilized in the telephone network as possible.
Similar considerations dominate the wireless communications arena. Because the frequency spectrum is limited, regulatory control of the frequency spectrum is typically much greater than in wired networks. The hurdles required to create a local wired network are much less than the hurdles required to create a local wireless network. Thus, portions of the frequency spectrum are assigned under strict regulatory control and signaling schemes must be employed that restrict any individual's use to their allotted portion of the spectrum.
Filters at the edge of the core telephone network limit voice grade bandwidth to about 3.3 kHz. That roughly translates to a top data rate of about 33.6 kbps for normal analog modems and up to about 53 kbps for some of the newer modem technologies. One emerging technology which promises to further increase the amount of information transferred over copper wire are digital subscriber links (DSL). Although a variety of different DSL exist, the ones that achieve the highest data rates are asymmetric digital subscriber links (ADSL). It is theorized that at least eighty percent of all telephone service subscribers are within 18,000 feet of a central office. It is anticipated that these customers will be able to get at least 1.5 megabits per second downstream (from the network to the user) and anywhere from 16 to 640 kbps upstream over ADSL equipped copper lines. At shorter lengths, ADSL speeds may increase.
ADSL uses frequency multiplexing, with one frequency band for regular phone service, another for upstream data, and a third for downstream data. Each of these bands uses efficient digital transmission techniques, such as quadrature amplitude modulation (QAM), to modulate a carrier with many different levels to get thousands of bits on a single frequency. Of course these speeds can only be achieved by removing the 3.3 kHz bandwidth limitation set by the voice grade filters.
Although ADSL seems to hold great promise, several problems exist. As previously noted, in order for ADSL to be a viable option, the 3.3 kHz bandwidth limitation must be removed. Furthermore, in many instances the particular pair of copper wires utilized must be tested and selected based on an adequate throughput. The increased bandwidth of ADSL is only available to those who live within a certain distance of a central office. Furthermore., ADSL requires costly equipment upgrades. Finally, although ADSL will work for certain types of subscribers, ADSL techniques provide no benefit to other networks, such as wireless communication networks. What is needed is a mechanism that may be used in a variety of environments to increase the transmission rate for a given bandwidth in order to transmit more data for a given bandwidth than is presently available.
In order to transmit more data in a given bandwidth, many systems have turned to compression techniques which compress a given block of data prior to transmission, transmit the data at a given data rate, and then uncompress the data at the receiving system. This is the approach taken, for example, by modems adhering to the V.42bis, which is one mechanism that has been used to increase the data rate down traditional analog phone lines to about 33.6 kbps. These mechanisms have the advantage of increasing the effective data throughput without increasing the actual transmission rate of the data. However, because such mechanisms do not increase the basic transmission rate of the data, there are limitations to how much additional throughput can be gained using such an approach. It would, therefore, be an advancement in the art to increase the basic underlying data rate down a fairly narrowband channel, such as an analog telephone line.
In order to increase the effective throughput over traditional analog telephone lines, a new generation of modems has recently been developed. These modems are the so-called. "56 k" modems. Although competing standards exist, in general 56 k modems attempt to increase the throughput by recognizing that rather than an analog modem connected to each end of a telephone line, typically the service provider end has a digital modem available. By placing a digital modem at the service provider end of the telephone line, the system may convert analog data to digital data prior to sending it to the service provider and the service provider can avoid converting digital data to analog data prior to sending it over the telephone network to the local modem. This results in greater throughput downstream but retains the traditional limitations on speed when sending data upstream.
It should be noted that all of these attempts to increase throughput do not address the fundamental problem of squeezing more information into a fixed bandwidth. Many, if not all, of the present proposals that claim to increase throughput are either based on compressing data prior to transmission or are based on either increasing the bandwidth of existing infrastructure or replacing existing infrastructure with new infrastructure having an increased bandwidth. It would, therefore, be an advancement in the art to provide systems and methods that dramatically increases the data throughput for a fixed bandwidth.